Natural Attenuation Software (NAS) is a screening tool to estimate remediation time frames for monitored natural attenuation (MNA) to lower groundwater contaminant concentrations to regulatory limits and to assist in decision making on the level of source zone treatment in conjunction with MNA, using site-specific remediation objectives. NAS development is funded by Southern Division, Naval Facilities Engineering Command (NAVFAC) and Naval Facilities Engineering Service Center (NFESC), and has resulted from a continuing collaboration between Virginia Tech, United States Geological Survey (USGS), and NAVFAC. NAS is designed for application to groundwater systems consisting of porous, relatively homogeneous, saturated media such as sands and gravels, and assumes that groundwater flow is uniform and unidirectional. NAS consists of a combination of analytical and numerical solute transport models implemented in three main interactive modules to provide estimates for 1) target source concentration required for a plume extent to contract to regulatory limits, 2) time required for non-aqueous phase liquid (NAPL) contaminants in the source area to attenuate to a predetermined target source concentration, and 3) time required for a plume extent to contract to regulatory limits after source reduction. Natural attenuation processes that NAS models include advection, dispersion, sorption, NAPL dissolution, and biodegradation. NAS determines redox zonation, and estimates and applies varied biodegradation rates from one redox zone to the next. With the successful development of the initial version of NAS and application of NAS to several contaminated sites, several critical needs had been identified. Software enhancements have been implemented to improve NAS and maximize its utility for site managers.

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